Rotary impact tool



Oct; 6, 1959 w. scHwENK ROTARY IMPACT T001.

2 Sheets-Sheet 1 Filed Jan. 7, 1958 is? um 4 4,

w. scHwENK 2,907,239

Oct. 6, 1959 ROTARY IMPACT TOOL 2 Sheets-Sheet 2 Filed Jan. 7, 1958 v/NVEN'OR nwmmmwn Wh United States Patent@ i ROTARY IMPACT TUOL WillySchwenk, Ludwigsburg, Germany, assignor to Robert Bosch G.m.b.H`Stuttgart, Germany Application January 7, 1958, SerialNo. 707,596 Claimspriority, application Germany January 8, 1957 1o Claims. (C1. :s1-52.3)

The present invention relates to power tools.

More particularly, Vthe present invention relates to power tools whichare adapted/to trn elements such as screws,` bolts andthe likenot on1y"with :al steady continuous turning'force but also with rotary impactforces when the` workpieces sucht as the bolts o r the like encounterresistances' to` turning. y j

v Conventionall power tools 'ofthis type4 are subjected to severestresses resulting from .the rotary impact forces, and it does notinfrequently happenthat partsv of `,such power tools, suchas parts of.the motors thereof, break orbtherwise fail due tothe impact forces.

One of the objects of the present inventionisto providea power tool ofthe above Atype, withVa-structure which will reliably shield the drivingvmotorofrthe tool from relativelywsevere forces to which the yinrpactprovidingv structure of the tool is subjected.

Another object of thepresent invention is to Vprovide a handJ power toolof the above type which will operate smoothly which is capable of being.comfortably held the operator. h Y h A further object of the presentinvention is toprovide a power tool of the above typewhich is capable ofautomatically counterbalancing and compensating forcesset up bytheimpact producing structure.

,An ladditional object of'the present invention: is'to provide a powertool capable of accomplishing the above objects and atthe same timecomposed of simple, ruggedly constructed elements which are reliablel inoperation.

With'the above objects in View the present invention includes in a powertool a rotary driven means which is adapted to cooperate at leastindirectly with a workpiece for turning the same and a rotary impactmeans which cooperates withl the driven'means for turning the vsame andfor drivingthe driven means with rotary impact blows when the drivenmeans encounters more thanV a predetermined resistance to turning. Theltool of the invention further includes a drive means Land, in accordancewiththe present invention, aV yieldabletransmission means whichcooperates with the drive means and Ythe rotaryrimpac't means fortransmitting a `drive from the drive means to the rotary impact "means,The yieldable transmission means of the present invention yields toshield'thedrive means from relativelysevere forcesto which the rotaryimpact means is subjected v"dining-operation ofthe power tool.`

The novel features which are considered as characteristic fortheinvention are set forth in in particularfin' the appended claims. Theinvention itself, however,"both yas toits constructionand its methodof"operation,fto

gether fwith additional objects and advantagesthereof,`

will be best iun'der'stood from thefollowing descriptionof specificembodiments when read inv connection withfthe F 1 isla sect1nalelevational view ofone-embedii 2,907,239 Patentedzorct.: 6, 1 959Fig. 2 is a sectional elevational view taken along'l-ine II-II of Fig. 3in the direction of the arrows and showing part ofthe yieldabletransmission means of the present invention; Y f v Fig. 3` is asectional plan view taken alonglline III- III of Fig. 2 in thedirectionof the arrows; I

Fig. 4 shows one type vof slip clutch lamination '.used in the structureof the present invention;

Fig'. 5 shows another type of slip clutch lamination; and

Fig. 6 is a sectional elevational view of another ,embodiment of. apower tool accordingto the present invention. f ff Referringto Fig. 1,the hand power'tool includesV a drive motor housing l1 cast from alight. metal such as aluminum, and thestator lamination's l `of anelectric 'n iotor. are vfixed ign-the housing.. 1. These lminationscarry a `stator .coil 3jcap'able of`being connectedelecvtrically throughthe cable 6 with a source of three phase alternating current uponactuation by the operator of the finger-operated switch 4 which closes asuitable switch located in'the handle 5. Themotor shaft 7 carries Iashort circuit armature 8 which turns with the ,shaft 7 when .the coil 3is energized. The shaft 7. carries` the inner races 9 and 10 of a pairof ball bearings 'whose outer races are supported by rthe right end wallof the yhousing 1, as viewed in Fig. l, and by thecover'11 of thehousing 1, respectively, .this cover 11 being fixed ,to andextendinginto the housingV 1 through the left open end thereof, as viewed in Fig.1.

Onv the other side of thecover 11 from the driving motor is located atransmission shaft 12 of a transmission means, this shaft 12 iixedlycarrying a relatively large gear 13 which meshes with a pinion 14 formedat the left free end of the shaft 7. The shaft 12 is formed with apinion .15 which meshes with a gear 16 which has an elongatedtubular hubportion which receives a portion of the drive shaft 17, the gear 16being freely turnable with respect to drive shaft 17.` The opposite endof the drive shaft 17 extends slidably into an Vaxial bore of a rotarydriven means 18 which is freely turnable 'with respect to the driveshaft, so that the driven means 18 and gear 16 serve as bearings for thedrive shaft 17. A rotary impact means 19 is formed with a bore 28through which the drive shaft 17 also slidably extends. Thus, the rotarydriven means 18 and rotary Aimpact means 19 are'freely rotatable andaxially shiftablek on the shaft 17.

The driven means 18 is supported for rotation by a brass bearingsleeve20 carried by a housing portion 21 between which and the housing V1 theycover 11 is clamped. Thus, the housing 21 forms together with cover 11and housing 1 a support means for the structure of the invention. Thedriven means 18 has an outer end provided with an end portion 22 ofsquare cross section adapted to cooperate with an unillustrated socketwrench which in turn cooperates with a bolt or the like for transmittingthe turning of the driven means 18 to such a bolt. Thenopposite end ofthe driven means 18 forms the Vdriven half of a dog clutch and isYprovided `with a clutch dog 23. The end of the rotary imdriven means 18to the lrest position thereofshown in Fig. 1. When the operator pressesthe tool to the left, as viewed in Fig. 1, the spring' 25` becomescompressed and the housing 21 shifts to the left along the driven means18.to cause the halves of the dogclutch tocome into mesh with eachother.

"Ihe rotary"iiripa'ctv means 19, except for its left end wall which isformed with the bore 28, is of tubular contiguration, and the annularsurface which deiines the bore 28 is formed with a recess 31 directedtoward the axis of lbore 28. vThe `recess 31'is'in the 'form of an axialgroove.` The AVouter surface `offdrive` shaft 4117 `is `formed with a'helical'groove 29'inclinedat approximately-45D 'to the axis of shaft 17and having a part directed toward the "recess 31. YAinotion transmittingelement in the form of. a spherical 'ball member 30i`slocated partly inthe "groove k29 and ipartly'in therec'ess 31.

A sleeve 32 coaxially surrounds 'the shaft A17 and extends lslitlablyinto -the "openend 'of 'the rotary impact means 19, and thissleeve .32has a reduced tubular Vend portion'jkeyed to `the s'haftrj17, vsothatthe l"sleeve 32 ro- 'ta'tes jvviththeshaft l17. Thereduced 'portion33 of the sleeve 32 axially guides the races 34 and 36 of a thrustV'3e':'a. 1"in'g,''and'the right'enldjof a'coil spring 35, as viewed .inFig. li'abuts lagainStfthe lrace 34. The other race pfengages apairof'push pins l33 'which are parallel` to shaft ,117"an`d arranged v-atAdiametrically 'opposed sides 'therci' These jpins extend fslidab'lythrough a pair of bores, respectively, formed in an annular wall portion'of sleeve 32. The ends of the pin's38distant Yfrom race 36abut'againsta 'pressure ring A'40. The outer surface of the isleeve'32 is providedwith a shoulder whichcarriesithevinnerracelll of a ball bearing,"theouter race 42j of which is carried in part by the housing 21 and inv'part by a partition 43 'between' wall 11 and hous- .ing 2v' 1^' .V l.Y l K y( l *l 'As is'apparent from Figs. 2 and 3 theelongatedtubularhub` portion45 of gear 16 gextend's into and is'surrounded vvbythe'sleeve 32. In the .space between the tubular hubporltion V45 and theinner surface 44 of'sleeve 32 are Alocated four annular slip clutchlaminations 50 and five slip clutch laminations 51 of a vslightlysmaller outer diameter thanthe lminations 50. Each'of lthe laminations50 is provided at its outer periphery with three projections 52'spaced12.0 apart from each other while each of the laminations 51 is providedat its inner periphery with three projections 54 also spaced 120 apartfrom eachother. The inner peripheries 53 of thelaminations 51 as well asthe inner .pe'ripheri'es 'of the laminations 50 surround the .tubular`hub portion 45, -and the two 'sets of laminations are. arranged withthelaminations of one setalter'nating with those 'of the other set. vTheyprojections 54 extend'into 'axial grooves 55 formed-in the hub portion45 while projections 52 extend into axial grooves '56 formedat the innersurface 44 of sleeve 32. An end 'ring 57 is located in sleeve 32 andprevented from moving out 'of the same by a snap ring 58.

In order to guarantee that the above-described transmission means willtransmit the drive from the motor to therotary impact means even in therest position of the parts shown in Fig. 1, the. length and compressionof the spring 35 in the position of `the part shown in Fig. 1

`is 'sufficient to enable this spring to act through the thrust bearing34, ..35 and push pins 38 to maintain the Islip clutch laminations 350and "51 in suicient frictional 'engagement with each other fortransmitting the drive from 'gear 16to shaft 17.

The above-described structure operates as follows: lfAsn'was' mentionedabove, when-the .operator presses -on the tool ,thedog clutch halveswill mesh. Thecom- .pressed spring .35 vwillholdthe rotaryimpact meansV19 ,in its illustrated voperating position with the ball .30 locatedat'thev, left end -of the -inclined'groove 29. Thus, .'thedriveistransmitted from ,shaft 17 'through element 30 toimpact :means-19 andby engagementy of dogs 24 and T23fto`the1driven'xrneans 18... vInthisway `rotationof the driven means 1`8willcontinue untilthe driven meansencounters a prede'termined 'resistance'to turning which is ygreatenough to prevent 'the `rotary impact means from turning tge'he'rwiththedrive shaft 317 spfthat the latter turns ahead of the impact member 19.At the moment when the rotary impact member 19 stops turning with shaft17 the ball 30 rides along the groove 29 to shift the driving half ofthe dog clutch together with the remainder of the rotary impact meansaxially away from the operating position shown in Fig. 1 to aninoperative disengaged position while simultaneously compressing thespring 35 further. Once the dog 2,4 slips past the dog 23 the spring 35expands and shifts the impact member 19 back to its operating positionwhile cooperation of hall 3% and groove 29 accelerate the angularturning of the impact member so that it catches up with the shaft 17,and after reaching its operating position the continued turning of theimpact member 19 causes its dog 24 to strike against the dog 23 toprovide an impact blow whose energy is transmitted to the head of theworkpiece such as a screw or the like which is being turned. Thisworkpiece is thus turned further while absorbing the force of the rotaryimpact blow. Then the above operations are repeated. Y v

v Inasmuch vasY the rotary impact means is sharply braked at eachimpact, the shaft 17 is loaded or stressed in a spasmodic, jerkymanner'as soon as the ball member 30 again 'cooperates with the groove29 to accelerate the impact member 19 in the axial direction. Thisspasmodic stress is to a verygreat extent shielded from the geartrain`13--16 and the armature of the driving motor .by momentaryslipping of the slip clutch in the sleeve 32, so that the'structureprovides between the driving motor and the drive shaft 17 a yieldabletransmission means capablev of yielding 'to prevent severe forces frombeing transmitted back through the transmission means to the drivingmotor.

A particular advantage of the above described structure resides in theplurality of functions performed y,by the spring 35. Not 'only does thisspring urgev the rotary impact member 19 to its operating position butthis spring additionally serves to press the slip clutch laminationsagainst each other. Moreover, the spring 3S is in its least compressedcondition at impact between the clutch -dogs 24 and 23* so that theleast pressure is exerted on the slip clutch elements at this time. Asthe drive shaft y17 continues to turn immediately after impact thespring 3S becomes compressed to an increasiingextent so asl to graduallyincrease the pressure with which the Vslip clutch 'laminations engageeach other so as to guarantee on the one hand transmission of the drivethrough the transmission means from the driving motor to the drive shaftand on the other hand to pro? videa smoothly operating tool.

I'he embodiment of the invention which is illustrated in Fig. 6 issimilar to that of Fig. 1 except for the yieldable part of the'transmission means. Referring to Fig. 6, the tool yillustrated includesa motor housing connected to a handle '111 and closed by a cover V112whose periphery is clamped between the motor housing 110 and the housing113. Themotor Ywithin the housing 110 is similar to that of Fig. 1 andincludes an armature shaft "114 formed with a pinion 115 at its vfreeend. The pinion 11S meshes with a gear 1116 fxedly carried by thetransmission shaft 117 which isprovided with a pinion portion v118 whichmeshes with a gear 120 having an -elongatedtubular'hub portion 123 andadjacent-the teeth of the gear 120 a cylindrical portion 121 rotatablein a bearing of the .partition 122, the gears 115, 116, 118 and' 12)forming part of the-transmission means. Y. .,-W g

The Vrotary impact means 126l is arranged in the housing 113coaxially-with gear12il)l .andfhas arleft relatively :portion whichforms withthe `dogs 1275and 128 integral therewith the driving halfuofhadog clutch Iadapted, 'to` cooperatev with the drivenuhalfof thedog`clutch -formed by the .right part of the rotary driven means 129 andincludingthe'clutch dogs 13!) and?131.

75 When the two clutch halves mesh the clutch dogshave sucient angularclearance to permit the driving clutch half to turn freely through apredetermined angle before striking against the driven half of theclutch.

The rotary impact member 126 is formed in portion 140 thereof with anaxial bore 141 through which the drive shaft 132 slidably extends, and acoil spring 133 is coiled about the drive Ishaft k132 and serves as aspring means for urging the rotary impact member 126 to the operatingposition shown in Fig. 6. The annular surface which denes the bore 141is formed with a recess 145 in the form of an axial groove, receivingpart of transmission element 134 in the form of a 'ball member, theother part of which extends into the inclined groove 135 formed in thedrive shaft 132, part of the inclined groove always beingdirectedvtoward .the recess 145. 'I'he drive shaft 132 is formed with anaxial bore 146 open at the right end o f the drive shaft and slidablyreceiving through this open right end the elongated tubular hub'portion123 of the gear 120. In this way the hub portion 123 serves as onebearing for the drive shaft 132, and the other bearing is formed by thelaxial bore 138 of the driven means 129 which slidably receives thereduced portion 139 of the drive shaft.

A ring 142 is freely shiftable on the drive shaft, and the right end ofspring 133 bears against this ring. The ring 142 is pressed by thespring 133 against an integral collar portion 143 of the shaft 132. Thering 142 has a cylindrical periphery 144 slidably engagingV the innersurface of thetubular portion of the rotary impact member 126. In thisway the ring 142 serves to guide.

the rotary impact member 126 at its end distant from the wall140.

A coil spring 148 is located in the communicating back to the drivingmotor.

axial bores 138 and 149 of the driven means 129 and drive shaft 132,respectively, and serves as a spring means for yieldably holding thedriven means .129 in lits illustrated rest position. .y

In order to transmit the drive from gear 120 to-the drive shaft 132,there is provided in the embodiment of Fig. 6, in accordance with thepresent invention, a torsion bar 150 having enlarged ends which arerespectively splined to the gear 120 and the drive shaft 132 in themanner shown in Fig. 6, and the splned connection is such that the shaft132 is capable of shifting axially with respect to the torsion bar 150,the latter carrying at its right end a washer which rests on the rightface of gear 1 20, as viewed in Fig.' 6, so that the torsion bar 150cannot move to the left beyond the position shown in Fig. 6.

The embodiment of Fig. 6 is set into operation by actuation of thefinger switch 151 which closes the circuit to the motor, and, of course,a socket wrench connected to the end portion 147 of square cross sectionof the driven means 129 cooperates with a bolt or the like -to beturned. When the operator presses to the left to compress the spring148, the driven clutch half will move into mesh with the driving clutchhalf, and the tool will turn the screw until there is suiiicientresistance to prevent the rotary impact means 126 from turning togetherwith the drive shaft. The latter then continues to turn, and element 134cooperates with groove 135 to move the impact member 126 to the right,as Viewed in Fig. 6, to an inoperative position, while the spring133 issimultaneously compressed to an increasing extent. After the clutch dogs127 and 4128 become disengaged from and ride over the dogs 130 and 131,the` spring 133 expands so as to axially shift the rotary impact member126 back to its operating position while simultaneously angularlyaccelerating the impact member so that it catches up with the driveshaft 132, and then the driving clutch half engages the driven clutchhalf with a rotary impact blow whose energy is transmitted to theworkpiece. The shock at impact is absorbed to a very large extent by thetorsion bar 150 which prevents the severe forces from being transmittedback to the driving motor. The above cycle of operations is repeatedtwice during each revolution with the embodiment of Fig. 6 and onceduring each revolution with the embodiment'of Fig. 1. It is apparentthat the embodiment of Fig. 6 also provides a yieldable transmissionmeans Which includes a torsion bar rather vthan a slip clutch to preventtransmission of severe forces A further advantage of the embodiment ofFig. 6 is that the drive shaft 132, the spring 133, and the rotaryimpact means 126 form a self-contained system supported by the bearingsprovided by hub portion 123 and bore 138 in cooperation with the driveshaft and axially shiftable in a downward direction, as viewed in Fig.6, while compressing the spring 148. As was mentioned labove inconnection with Fig. l, during expansion of the spring 133 to shift therotary impact means back to its operating position, the rotary impactmeans accelerates in a jerky, spasmodic manner, and. the stresses set upby this action are compensated in the embodiment of Fig. 6 by momentaryshifting of the driveA shaft to the left as viewed in Fig. 6, whilecompressing the spring 148, which immediately thereafter expands toreturn the drive shaft to the axial position shown in Fig. 6.

This operation guarantees quiet running of the machine and enables it tobe comfortably handled by the operator as a result of the smoothoperation. Y l ItY will be understood that each of the elementsdescribed above, or two or more together, may also find a usefulapplication in other types of power tools differing from the typesdescribed above.

While the invention hasbeen illustrated and described as embodied inrotary power tools, it is not intended to be limited to the detailsshown, since various modiiications and structural changes may be made`without departing in any way from the spirit of the present invention.Y, ;A

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention thatothers can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations shouldv and are intended to becomprehended within the meaningand range of equivalence of the followingclaims.

What is claimed as new and desired to be secured by Letters Patent is:

l. A power tool comprising, in combination, support means; rotary drivenmeans having an axis of rotation and supported by said support means forrotational and axial movement with respect to said axis, said drivenmeans having an outer exposed end adapted to cooperate at leastindirectly ywith a workpiece to be turned bythe power tool and anopposite end in the form of the driven half of a dog clutch; a rotaryimpact member coaxial with said driven means and having a driving end inthe form of the driving half of said dog clutch directed toward and glocated adjacent said driven half of said dog clutch, said rotary impactmember' being formed with an axial bore defined by an annular-surfaceformed with a recess directed toward the axis of .said impact member;first spring means cooperating with said. driven means for yieldablymaintaining the latter in an outer rest position, said driven meansybeing axially shiftable by the operator against the force of said firstspring means inwardly to an operating position where the driving ,anddriven halves of said dog clutch mesh for transmitting a drive fromsaidrotary impact'member to said rotary driven means, said clutch halveshaving one with respect to the other an angular clearance providing free'turning of said driving clutch half with respect to said driven clutchhalf until' impart arotary impact blow thereto; a drive shaft extendingslidably through s'ai'd 'axial bore of'said impact member coaxially withthe latter and said rotary driven means; said' drive 'shaft being'formed'withr a helical groove,.part of 'which is directed toward saidrecess of said impactm'ember'; a motion transmitting element loi catedpartly in said recess "and partly in said 'groove for transmitting'rotation 'of saidl 'drive shaft to said'impact member 'and for movingalong Vsaid groove to shift'said rotaryimpact member away from 'said'rotarydriven means rwhen the 'resistance to turning ofthe latter isgreat enough to prevent said driving clutch half from turningat'the'sa'me speed as said'drive shaft, said groove 'being long enoughto providewith said motion transmitting el'ement an'axial movement ofsaidfdriving clutch half yto a 'position'out of driving engagementwithsaid driven clutch half 'when said 'driven means encounters saidresistance; second springI means cooperating'with `s aid rotary impactmember for urging vthe latter toward `saiddriven means and `for`yieldably resisting movementjof `said driving clutchV 'half out` ofengagement'with said driven clutch half; 'adriving'motor carried by saidsupport means; and yieldable transmission means connecting said drivingmotor 'with said driveshaft for rotating the latter, said transmissionmeans yielding to shield said drivingl'motor from relatively severeforces to which'said rotary impact member is subjected during operationof the power'tcol. A'power tool as recited lin claim l and whereinsaidyieldabl'ertransmission means includes aslip clutch which slips toabsorb said relatively sever'e forces and vprevent them'from beingtransmitted back' to said driving motor. l '3. `A power tool as recitedin' claim l and wherein said yieldable transmission `means includesaltorsionbar which twists to absorb said relatively severe forcesandpievent `thernfrom -being transmitted back-to saidl drivingmotor. r4.lA'po'wer tool as recited infclaim 'l andwh'erein said yieldable4transmission means 'includes two sets of slip clutch'laminationsarranged with the laminations of one setfalt'ernatin'gfwiththose of the other and beingin frictional engagement therewith, saidsets of laminations being respectively connectedwith said drivingmotorand'drive shaft through additional parts of said transmission means for'friotionally--transtnitting the Vdrive `from said driving motortosaiddrive-shafhlsaid 4sets of laminatio'ns slipping with A"respecftltoeach other-to absorb said relatively severe 'forces "and'prevent them-frorn being transmitted vback tosaid driving motor. Y Y 5. vA power4tool-as recited in claim 4 and wherein said second spring means jactsonlsaid Vlarninations vfor urging 'them against eachother and forvi'naintaini'ng them in frictional engagement.

6, A power tool as recited in claim 4, a thrust bearing located betweensm'd second spring means and slip clutch laminations and urged by ksaidsecond spring means toward said laminations and a plurality of push pinslocated between and engaging said thrust bearing and the end laminationnearest thereto so that said second spring 4means acts through saidthrust bearing and pins for maintaining said laminations in frictionalengagement with each other.

7. A power tool as recited in claim l and wherein said yieldabletransmission means includes a sleeve coaxially surrounding 'and spacedfrom said shaft and having an end portion of reduced diameter fixedthereto, so that said sleeve rotates with said drive shaft, said sleevehaving an inner surface directed toward said shaft and formed with l Vanaxial groove; said yieldable transmission means also including a gearcoaxial with said drive shaft and having anelongated, tubular hubportion in which part of said shaft is jfr'eely turnable, said hubportion extending into s'aid 'sleeve "andhaving an ontersurface formedlwith Van axial groove directed'toward said inner sleeve surface, aplurality of annular slip clutch laminations -locatedfnext to 'eachotherin saidsleeve surrounding said tubular hub portion, 'aplurality of 'saidlaminations lhaving outer vprojections'lo'cated in said groove of saidsleeve and a second plurality "of laminations alternating with saidfirst-mentioned plurality of laminations, frictionailyengagin'g thesame, and having 'inner vprojections Vlocated in said hub groove, saidsecond spring means acting on said laminatio'ns toriurging lthe sametogether so as to maintain them in, frictional engagement 'and saidldriving motor driving said lgearand said sleeve and drive shaft throughs'aid laminat-ions,"theglatter slipping to preventV said severeyforc'esfro'rn being transmitted back -to'saidfdriving motori "8, 'Apower tool as 'recitedinclairnl said 'yieldable transmission meansincluding fa '-gear ft'rain operatively connected with said drivingmotor-andterminatin'gfin an end gear coaxial withvsaiddrive shaft and `atorsion bar coaxial with saidjend gear and drive shaftand 1havingopposite end portions `respectively splined to said end gear and driveshaft so that :the latter is driven by said gear train throughsaidtorsion "bar, the latter yieldably twisting to prevent said 's`evereforces 'from being transmitted-back tofsaid driving motor.

9. A power tool'as recited 'in claiml 8, said second spring-means beingi'n the form of 'a coil spring coiled aboutsaid drivey shaft,'=actingwith one end against said impact member for urgin'gthe lattertoward saiddriven 'means'a'nd :acting'with its opposite end against a portion ofsaid drive shaft, the latter being movable during move- 'ment offs'aid-fdriving'half ot" said dog clutch in one axial ldirection lout. ofengagement with said driven clutch half in an. opposite .axial directionfor'opposing forces of said rotary impact membervduring,'disengagernerit of the dog clutch.

'10. A power-tool'as recited in claim-1, said drive shafthavingan-elongated hollowzportion a'nd an open'end distant 'from saiddriven means and communicating with said hollow portion, and said driveshaft having an outwardly directed annl'arcollar portion, said secondspring means being=inthe`formof-a lcoil spring coiled about said driveshaft, acting with one end on said impact member tor 'urging thej-latter to-ward said driven means and acting vwith its other yend on'said collar portionof said vdrive `shatt',-"saidyieldableftransm'ission means 'including aV gear train "connectedoperativelywith said-driving motor and terminating in an end gearcoaxial Vwith said shaftand having an elongated tubular hub portion-extendingfreely into said hollow portion'of-said driyeshaft 'throughsaid open -endfthereoi anda torsion bar-located coaxially in said-tubular-hubgportion'and hollowportion of s aid drive shaft, having oneend splined-to'said end gear, `and having an opposite `end-splined .tosaiddrive shaft so `that said Vbar tvtnfststo preventsaidsevere forcesfrom being transfmittedback tosaid drivingfmotor. f

References vCited inthe le of this patent UNITED STATESVPATENTSjAmtsbergf ;i -n1 c Nov. 4,l 1941 2,533,703A -Wilhide-etval Dec. 12,1950 26625424' Burkhardt L Dec. 15, 1953 2,691,434 fim'erson" `f r Oct.l2, 1954 2,784,625 Maurer Mar. 12, 1957 .2g-821,276 t. "Reynolds Jan.28, 1958 Reynolds

